The development of hydrogen as an energy vector has been the subject of major research efforts for several decades. Interest in this field intensified in recent years because of the possibility that hydrogen could provide a source of renewable and entirely non-polluting energy for mankind, thus releasing it from dependence on fossil fuels and the ensuing environmental effects (the greenhouse effect, for example). For a general review of the subject see I. Dostrovsky, Energy and the Missing Resource, Cambridge University Press, (1988), while for a specific review of the use of hydrogen as an energy vector see C. J. Winter and J. Nitsch, Wasserstoff als Energietrager, Springer-Verlag (1986).
A very promising method of production of hydrogen by renewable energy consists of the dissociation of water by concentrated solar radiation. It is well known that at a sufficiently high temperature water will spontaneously decompose into its constituents. The temperatures required are high, well above 2000 degrees centigrade, but are readily achieved through concentrated solar radiation and it can be shown that at such high temperature water vapor decomposes to yield a gaseous mixture with an appreciable proportion of hydrogen. However, the separate recovery of hydrogen and oxygen from such gaseous mixtures raises some problems.
The methods of separation of a gas mixture which found wide application in isotope separation, such as gas centrifugation, thermal diffusion and mass diffusion (S. Villani, Isotope Separation, ANS (1976)), are clearly not suitable in their known configurations for separation of individual components from the very hot gas mixture formed in the process of direct splitting of water.
A method for separation of the hot products of water splitting that has been proposed in the technical literature (E. A. Fletcher and R. L. Moen, Science, 197:1050 (1977)) is gas diffusion through a porous wall under a Knudsen flow regime. The gas flow through the porous wall is defined as a Knudsen flow if the pressure in the gas is sufficiently low, so as to maintain the molecular mean free path in the gas larger than the wall mean pore size (S. Villani, Isotope Separation, ANS (1976)).
When part of the water dissociation products formed in a hot reactor are extracted from the reactor by diffusion through a porous wall under Knudsen flow conditions, while the remaining dissociation products bypass the porous wall on their way out of the reactor, the gas stream that diffuses through the porous wall is enriched in light gas components while the bypassing stream is depleted of light gas components.
It should be noted that when leaving the reactor, both streams are at a very high temperature. In order to obtain a high yield of hydrogen for a given amount of solar radiation (i.e. high hydrogen production efficiency) it is necessary to recover as much as possible of the thermal energy contained in these hot gas streams. A process has been proposed (unpublished) in which water is heated to a high temperature in a reactor chamber whose walls include two membranes. One membrane is a porous ceramic wall which under appropriate flow conditions, it is preferentially permeable to hydrogen. The other membrane is a solid-oxide membrane, such as an yttria stabilized zirconia membrane of the kind used in fuel cells, through which oxygen can pass preferentially by ionic conductance (J. E. Noring et al., Energy, 6: 109.(1981)). In accordance with that process, part of the hot products of water dissociation is extracted from the reactor by diffusion through the preferentially hydrogen permeable membrane and another part is extracted by diffusion through the membrane permeable to oxygen. The remaining dissociation products bypass the two membranes on their way out of the reactor and are recirculated to the reactor by some pumping means, such as a steam injector actuated by high pressure make up steam, and in this way the sensible heat carried by the recirculated products is recovered.
The solid oxide membranes used in this process have a high ionic resistance and consequently a very low oxygen permeability, which constitutes a serious drawback of the process. Moreover, in order to obtain efficient gas separation by diffusion through a porous wall, the pore size must be at most of the order of magnitude of the molecular mean free path of the gas mixture. This requirement places a limitation on the maximum gas pressure inside the chamber containing the gas separation membrane, which is yet another drawback. By way of example, assuming a pore diameter of the order of 15 microns and a gas temperature of 2500 K, the solar reactor chamber pressure should preferable be below 0.04 atm.
A primary object of the present invention is to provide an effective method for the separate recovery of gases of different molecular weight from gas mixture by gas diffusion.
Another object of the invention is to provide a method for the separate recovery of hydrogen and/or oxygen from a gas mixture resulting from direct thermal splitting of water, without recourse to preferentially oxygen permeable membranes.
It is yet another object of the invention to provide a method of the kind specified with an improved heat regime.